To an increasing degree, clean rooms are required in plants where precision instruments or the requisite parts for such instruments are manufactured. Also, the demands placed on the purity of the incoming and circulating air used in industrial processes are becoming increasingly more stringent. It is essential to have air that is purified of dust and microorganisms, and air filter systems having different designs are used. In this connection, cartridge filters are frequently used, which have a fold pack that is oriented perpendicularly to the direction of flow or a plurality of fold packs arranged in a V-shape, which are inserted into one frame. The fold pack and the frame are sealed from one another by a sealing material. As a fold pack, an insert made of accordian-folded filter paper is used. When working with filters for fine particulate matter, a filter fleece can be inserted as sealing material. However, in the case of filters for suspended particulate matter, plastics, which are mostly processed in liquid form, are used.
The frame gives the filter the requisite stability, protects the fold pack from damage, and allows the filter to be inserted into a receptacle system provided for it. The finished filter, together with the frame, is installed on site in a holding receptacle of an air filter system. The air filter systems frequently have a multistage design and are composed of series-connected filters, for the most part of different filter classes, the separation efficiency of the filters typically increasing in the direction of air flow. When the filters of any one stage are installed in a filter system, in each case in a separate holding receptacle, the filter system must be sized to have equivalent dimensions. This entails higher capital costs as compared to a single-stage system. For this reason, efforts are also directed toward achieving a most compact possible design of the two-stage or multistage filter systems. In practice, in a two-stage filter arrangement, the prefilter and the main filter are installed together in only one filter receptacle. The prefilter is positioned loosely against the main filter, and, once inserted in the filter receptacle, both elements are held securely by the latter. The sealing action between the prefilter and main filter is provided by a peripheral seal that is placed on both filters. Since the service life of the prefilter is typically shorter than that of the main filter, problems arise when replacing the prefilter. The problems are further aggravated in filter systems which are only accessible from one side, since the prefilter and the main filter can only be installed from one and the same side into the holding receptacle.